1. Field of the Invention
The present invention relates to a casting apparatus and to a method for producing ultra large, thin-walled components and, more particularly, to a molten metal injector system for producing ultra large, thin-walled components.
2. Description of the Prior Art
The manufacturers of ground transportation vehicles, such as automobiles, sport utility vehicles, light trucks, vans, buses and larger capacity trucks, have made major efforts in recent years to reduce vehicle weight. Weight reductions reduce harmful atmospheric emissions and increase fuel efficiency of ground transportation vehicles. Presently, a majority of the body components for ground transportation vehicles are formed from individual steel components that are assembled via resistance spot welding. For example, the floor pan frame of an automobile is normally constructed from a number of individual steel stampings that are spot welded together. It would be advantageous to produce body components for ground transportation vehicles, such as the floor pan frame of an automobile, as a single ultra large casting. As a result, the costs associated with producing multiple steel stampings and then assembling the stampings may be eliminated. The same technology would also be suitable for components in the aerospace industry.
There are several known methods for producing thin-walled castings. Examples include: high pressure cold chamber vacuum die casting, premium sand casting, a level pour process practiced by Alcoa, Inc. for producing components for the aerospace industry and low pressure hot chamber injection. Low pressure hot chamber injection is particularly well-suited for producing components made from non-ferrous metals having a low melting point, such as aluminum, brass, bronze, magnesium and zinc.
A typical casting apparatus and method known in the prior art for the casting of low melting point temperature metal materials is disclosed in U.S. Pat. No. 4,991,641 to Kidd et al. (hereinafter xe2x80x9cthe Kidd patentxe2x80x9d). The Kidd patent discloses an apparatus that includes a supply tank configured to contain molten metal alloys and a cylinder in the tank having at its base a connection to an injection passageway that leads through the tank to a casting die located outside the tank. A piston reciprocates in the tank, which allows the molten metal alloy to be drawn into the cylinder and forced through the injection passageway to the casting die. A control system for the piston controls the speed of the piston in the cylinder when the molten metal alloy is fed to the casting die. Other similar prior art casting devices are disclosed in U.S. Pat. Nos. 5,082,045 to Lambert; 5,181,551 to Kidd et al.; and 5,657,812 to Walter et al. Each of the devices disclosed in the foregoing patents includes a reciprocating piston that injects molten metal into a mold cavity during the downstroke of the piston.
Piston arrangements such as those disclosed by the Kidd patent have several disadvantages. For example, the use of reciprocating pistons that inject molten metal to a casting die during the downstroke have a tendency to disturb the metal oxide film surface of the molten metal alloy contained in the supply tank. Consequently, undesirable metal oxides and/or air bubbles are often injected into the casting mold along with the molten metal alloy, thus resulting in an inferior casting. Even if the metal oxide film surface of the molten metal alloy is not substantially disturbed, metal oxides sometimes form in the piston cylinder during the downstroke of the piston.
Accordingly, it is an object of the present invention to provide an apparatus and method for the casting of inexpensive, thin-walled components. In addition, it is an object of the present invention to provide an apparatus and method for casting thin-walled components of such size and complexity that traditional stamping assemblies made from multiple stamped components could be replaced with a single, thin-walled component. Finally, it is an object of the present invention to generally overcome the deficiencies of the prior art such as those described herein in connection with the Kidd patent.
The above objects are accomplished with a molten metal injector system according to the present invention. The molten metal injector system of the present invention includes a holder furnace for containing a supply of molten metal having a metal oxide film surface. A casting mold is supported above the holder furnace and has a bottom side facing the holder furnace. The mold defines a mold cavity for receiving the molten metal from the holder furnace. A molten metal injector is supported from the bottom side of the mold and projects into the holder furnace. The injector is in fluid communication with the mold cavity and includes a piston positioned within a piston cavity defined by a cylinder for pumping the molten metal upward from the holder furnace and injecting the molten metal into the mold cavity under pressure. The piston and cylinder are at least partially submerged in the molten metal when the holder furnace contains molten metal. The cylinder further includes a molten metal intake for receiving the molten metal into the piston cavity. The molten metal intake is located below the metal oxide film surface of the molten metal when the holder furnace contains the molten metal.
The molten metal intake is preferably located sufficiently below the metal oxide film surface when the holder furnace contains molten metal such that the metal oxide film surface remains substantially undisturbed during pumping of the molten metal from the holder furnace to the mold cavity. The piston may be oriented substantially perpendicular to the bottom side of the mold and movable through a downstroke and a return stroke. The injector may further include a lifting mechanism positioned above the metal oxide film surface when the holder furnace contains the molten metal. The lifting mechanism is preferably operatively connected to the piston for moving the piston through the downstroke and the return stroke. The molten metal preferably flows through the molten metal intake and into the piston cavity during the downstroke of the piston when the holder furnace contains the molten metal. During the return stroke of the piston, the molten metal received in the piston cavity is preferably pumped upward from the holder furnace by the piston and injected into the mold cavity under pressure.
The molten metal intake may be a valve configured to open during the downstroke of the piston and permit inflow of the molten metal into the piston cavity such that metal oxides are substantially prevented from forming in the piston cavity. In addition, the molten metal intake may be a gap formed between the piston and a tapered inner surface of the cylinder at a substantially full downstroke position of the piston. Furthermore, the molten metal intake may be an aperture formed in a sidewall of the cylinder and connected to the piston cavity. The aperture may be open for inflow of the molten metal into the piston cavity when the piston is in the substantially full downstroke position. A molten metal filter may be used to cover the molten metal intake for filtering and removing debris from the molten metal flowing into the piston cavity through the molten metal intake.
The piston and the cylinder are preferably made of a material compatible with molten aluminum alloys. The lifting mechanism may be a rack and pinion. The piston cavity may be in fluid communication with the mold cavity through a fill tube passing through the bottom side of the mold. A source of inert gas may be in fluid communication with the fill tube such that during the downstroke of the piston, the piston cavity is filled with inert gas flowing down the fill tube for substantially preventing the formation of metal oxides in the cylinder.
The present invention is also a method of operating a molten metal injector in connection with a supply of molten metal and a casting mold having a mold cavity. The method preferably includes the steps of: providing the supply of molten metal; providing the molten metal injector, with the injector having a cylinder defining a piston cavity housing a reciprocating piston, with the cylinder including a molten metal intake for receiving molten metal from the supply of molten metal into the piston cavity, and with the piston movable through a downstroke and a return stroke by a lifting mechanism operatively connected to the piston; supporting the injector above the supply of molten metal such that the cylinder and piston are at least partially submerged in the supply of molten metal, and such that the molten metal intake lies completely submerged in the supply of molten metal; moving the piston through a downstroke with the lifting mechanism; permitting inflow of the molten metal from the supply of molten metal into the piston cavity through the molten metal intake during the downstroke of the piston such that the piston cavity is at least partially filled with the molten metal; moving the piston cavity through a return stroke with the lifting mechanism; and preventing the inflow of the molten metal from the supply of molten metal into the piston cavity with the molten metal intake during the return stroke of the piston.
The method according to the present invention may also include the steps of: locating casting mold above the supply of molten metal such that a bottom side of the casting mold faces the supply of molten metal; supporting the injector from the bottom side of the casting mold; and placing the piston cavity in fluid communication with the mold cavity such that during the return stroke of the piston the molten metal received in the piston cavity through the molten metal intake is injected into the piston cavity.
Further, the method according to the present invention may include the steps of: providing the molten metal intake as a valve having a valve controller operatively connected thereto for opening and closing the valve; opening the valve with the valve controller during the downstroke of the piston such that the valve permits the inflow of the molten metal from the supply of molten metal into the piston cavity; closing the valve with the valve controller during the return stroke of the piston such that the valve prevents the inflow of the molten from the supply of molten metal into the piston cavity. The method may further include the steps of: supplying inert gas to the piston cavity during the downstroke of the piston for preventing the formation of metal oxides in the piston cavity; and filtering the molten metal flowing into the piston cavity through the molten metal intake during the downstroke of the piston with a molten metal filter.